Hydrogen sulfide gas sensor and precursor compounds for manufacture of same

ABSTRACT

Tungsten carboxylate compounds useful for coating interdigitated electrodes used in hydrogen sulfide gas sensors are disclosed. A method of coating electrodes with the compounds using a precise solution casting technique such as spin-coating or casting, dip-casting or spray-casting techniques is also described. Electrodes which are solution coated with the compounds may be used to fabricate superior quality chemiresistor sensors for use in hydrogen sulfide gas sensing devices by heating the carboxylates above 350 DEG  C. to decompose certain carboxylates to WO3 and others to sodium tungstate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 08/200,479, filed 23Feb. 1994, now U.S. Pat. No. 5,433,971, which is itself a divisional ofU.S. Ser. No. 07/934,920, filed 25 Aug. 1992, now U.S. Pat. No.5,321,146, which is itself a continuation-in-part of U.S. Ser. No.07/677,729, filed 29 Mar. 1991 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to hydrogen sulfide gas sensors. In particular,it relates to a chemiresistor coating for electrodes used in hydrogensulfide gas sensors.

2. Description of the Prior Art

Hydrogen sulfide is a toxic gas which has the ability to temporarilydeaden the human sense of smell. Therefore, there is an importantbenefit in being able to detect the presence of hydrogen sulfide gas inthe environment.

In addition to health concerns, the presence of sulfide during theproduction of photographic products may directly affect the quality ofthe product.

The extent to which one can control the sulfide content in either theatmosphere or in the photographic product manufacturing process dependson the ability to measure it. Therefore, the detection and quantitativeanalysis of sulfides, even at trace amounts (i.e., ng/ml), must beprecise.

A chemiresistor sensing device generally contemplates the use of a powersupply transmitting current through a sensor which contains asemiconductor material, such as a metal oxide. The semiconductormaterial behaves as a chemiresistor. A chemical influence can be causedby an ambient gas interacting with the semiconductor material and can bemonitored by a change in the resistance or conductance of the materialby the use of electrodes which transmit the change in conductance to amonitor or detector means, such as a voltmeter.

Chemiresistor gas sensors using semiconductor materials comprised ofthin film metal oxides, such as tungsten oxide, have shown goodsensitivity for detecting reducing gases, such as hydrogen, anhydrousammonia, hydrazine, propane, butane, methyl alcohol, ethyl alcohol andhydrogen sulfide (H₂ S).

Chemiresistor sensors which incorporate thin films of tungsten oxide asthe sensing material have been known to respond selectively andsensitively to hydrogen sulfide gas. The exposure of tungsten oxide tohydrogen sulfide gas results in a decrease in the resistance of thesensing metal oxide. A measurement of the decrease in the resistance ofthe sensing metal oxide can be used to determine the concentration ofthe hydrogen sulfide gas. Certain known chemiresistor sensors comprise aresistor layer, such as a heater resistor, an electrical connection tothe heater, a support layer, such as an alumina substrate, a conductorlayer (often composed of interdigitated electrodes) and a depositedchemical sensing layer most frequently comprised of tungsten oxide (Seefor example, Jones et el., U.S. Pat. No. 4,822,465).

The manner in which the tungsten oxide semiconductor material is appliedto the electrodes is of particular importance because the microstructureresulting from the method or technique of depositing the tungsten oxidelayer can affect both the selectivity and sensitivity of the tungstenoxide layer to hydrogen sulfide gas.

The sensors described in Willis et al., U.S. Pat. No. 4,197,089,describe hydrogen sulfide gas sensors with improved selectivity tohydrogen sulfide gas, which comprise a chemically formed sensor film oftungsten trioxide produced by decomposing a droplet of ammoniumtungstate contained in solution and deposited on the sensor. The patentalso discloses a physically formed sensor film of tungsten trioxidewhich is produced by sintering tungsten trioxide in the powder form onthe electrode surface.

One major disadvantage inherent in the above techniques is an inabilityto manipulate the microstructure of the film formed. Depositing powderedtungsten oxide and sintering the powder or by placing a drop of anaqueous solution containing ammonium tungstate over the electrodesfollowed by thermal decomposition are rather crude methods for thecreation of a film on the electrode. Uncontrolled microstructure of thefilm leads to unpredictable sensitivity and selectivity of the sensingfilm. The inability to manipulate the microstructure of the thin filmprecludes optimizing the sensitivity and selectivity for a given set ofconditions. Since the method used to deposit the thin film will dictatethe microstructure of the metal oxide film and, since the microstructureof the metal oxide film may determine the selectivity and sensitivitytoward the reducing gas of interest, the method used to deposit thesensing film is very important to its sensing abilities.

Another method for depositing thin films of tungsten oxide on electrodesis referred to in Jones et al., U.S. Pat. No. 4,822,465, which discusseswhat is known as a radio frequency sputtering technique. This techniquecontemplates a deposit of the sensing film by sputtering the film ontothe electrodes which are, in turn, supported by a substrate. One of theshortcomings of depositing sensing films by sputtering techniques ariseswhen dopants are added to the sensing film. If it is desirable for thesensing film composition to contain a dopant; it is preferred that thedopant be uniformly dispersed throughout the sensing compound to provideconsistence in the electrical properties of the film. This is difficultto achieve using sputtering. In addition, sputtering may yield mixturesin which there is either less than or more than the optimalconcentration of dopant in the sputtered thin film.

In addition, the radio frequency sputtering technique inherentlyintroduces varying levels of stress into the thin film which may effectthe sensing capability of the thin films. This stress results from theinability of the sputtering technique to deposit the sensing filmuniformly over the surface of the electrode. Conformance to irregularsubstrates is often poor with sputtered films.

All of the shortcomings of sputtering enumerated above could beameliorated by solution casting techniques. However, until developmentof the present invention, application of tungsten oxide thin films usingspin-casting (also known in the industry as spin-coating), dip-castingand spray-casting solution techniques (hereinafter collectively referredto as solution casting techniques), has been unavailable. Untildevelopment of the present invention, known technology was unable toprovide for the precise and uniform application of tungsten oxide filmsonto electrodes, because tungsten oxide is insoluble in the aproticsolvents used in solution casting techniques.

Thus, a need still exists for improving the application of thin filmmetal oxides and, in particular, tungsten oxide to electrodes containedin hydrogen sulfide and other reducing gas sensors.

It is an object of the present invention to provide tungsten compoundsthat are soluble in aprotic solvents, that can be solution cast and thatthermally decompose to provide tungsten oxide or sodium tungsten oxideelectrodes for detecting hydrogen sulfide.

SUMMARY OF THE INVENTION

In answer to these unmet needs, two genera of tungsten carboxylates aredisclosed which can be thermally decomposed to form tungsten trioxide(WO₃) and a third genus is disclosed which can be thermally decomposedto sodium tungsten oxides. The genera are readily soluble in severalcommonly used aprotic organic solvents, including aromatic and aliphaticsolutions, and can be applied to the electrodes contained in a hydrogensulfide gas sensor using a precise solution casting technique.

In one aspect the invention relates to compounds represented by FormulaI

    Na[OW(OOCR).sub.2 ].sub.2                                  I

wherein R is alkyl, alkenyl or aralkyl of 2 to 19 carbons. Thestoichiometry of the compounds is best represented by the empiricalformulae shown, but their actual structures can be monomeric, dimeric orpolymeric, as is well known in the art for tungsten carboxylates.Preferred subgenera include those in which R is alkyl or aralkylcontaining 6 to 10 carbons, particularly 1-ethlypentyl, 2-phenylpropyland 3-phenylpropyl.

In other aspects, the invention relates to a method for preparing sodiumtungsten Oxide by the thermal decomposition of compounds of formula Iand to a method for preparing the compounds of formula I by reacting analkali metal with an excess of a C₃ to C₂₀ acid to form acarboxylate-salt solution; and reacting the carboxylate-salt solutionwith a solution containing tungsten (VI) oxychloride in an aromaticsolvent to form a sodium tungsten carboxylate salt.

Because of the uncertainty in representing the structures of tungstencompounds, the invention may also be described as relating to a tungstensalt, soluble in aprotic solvents, prepared by the process consistingessentially of combining tungsten (VI) oxychloride with four equivalentsof sodium 2-ethylhexanoate and a large excess of 2-ethylhexanoic acid intoluene and refluxing for 16 hours.

In another aspect the invention relates to compounds represented byFormula II ##STR1## wherein n is an integer from zero to three, mostpreferably n is three.

In another aspect the invention relates to compounds of Formula III

    ClO.sub.3 W.sub.3 (OOCR).sub.2                             III

wherein R is alkyl, alkenyl, or aralkyl of 2 to 19 carbons, preferably Ris 1-ethlypentyl. As before, because of the uncertainty in representingthe structures of tungsten compounds, this aspect of the invention mayalso be described as relating to a tungsten compound, soluble in aproticsolvents, prepared by the process consisting essentially of combiningtungsten (VI) oxychloride with thirty equivalents of 2-ethylhexanoicacid and heating at 160° C. for 24 hours.

In other aspects, the invention relates to a method for coating anelectrode for use in a hydrogen sulfide sensor comprising the steps of

dissolving a compound of Formula II or III in a solvent to form atungsten carboxylate precursor solution;

depositing the precursor solution on an electrode using a solutioncasting technique to form a thin film coating over the electrode; and

heating the coated electrode so that coating decomposes to tungstenoxide.

In similar fashion compounds of Formula I are converted to a sodiumtungsten oxide coating.

A coated electrode, or a plurality of coated interdigitated electrodes,can be fabricated using the novel compounds by dissolving the precursorcompounds in a solvent to form a precursor solutions. The controlledcoating of the electrodes is then accomplished by coating the electrodewith the precursor solution using a standard solution casting techniqueof the type commonly employed for spin-casting or spin-coating,dip-casting or spray coating or casting. The electrode is then heated byconventional curing means to decompose the tungsten carboxylateprecursor which has been deposited thereon by the desired solutioncasting technique. The decomposition of the uniform thin precursor layerresults in a controlled uniform thin layer of tungsten oxide. Once thedecomposition occurs, the electrode coating is capable of reactinghighly sensitively and selectively to hydrogen sulfide gas.

In its broadest sense, the invention also encompasses a hydrogen sulfidegas sensor having an electrode coated with a tungsten oxide derived fromthermal decomposition of the novel precursor.

The object of the invention is to improve hydrogen sulfide gas sensors.

It is a feature of this invention to enable the fabricator of hydrogensulfide gas sensors to coat interdigitated electrodes and otherelectrodes with a precursor which, when heated, decomposes into tungstenoxide or sodium tungsten oxides.

One advantage of the present invention is the ability to coat electrodesused in hydrogen sulfide gas sensors more precisely, and thereby createa more consistent tungsten oxide or sodium tungsten oxide microstructureon the electrode.

A further advantage of the invention is the improved substrateconformity of the tungsten oxide or sodium tungsten oxide thin film overthe interdigitated electrodes. If the electrode surface were to containsmall indentations or protrusions, these imperfections could becompensated for by precisely applying the precursor compound usingsolution casting techniques.

A still further advantage of the invention is the improved rheology or"wetting ability" of the compound being deposited on the electrodesubstrate thereby making it useful in thin film spin-casting or coating,dip-casting and spray techniques, collectively referred to as "solutioncasting techniques".

A still further advantage of the invention is the ability to uniformlymix a dopant with the precursor compound and apply a uniform mixture ofprecursor and dopant through the use of a precise solution castingtechnique.

A still further advantage of the invention is the ability to reduce thestress of a thin sensing film which results from non-uniformapplication. In particular, it is noted that the stress of thin filmsdeposited by the solution casting technique is significantly less thanthe stress measured in thin films deposited by use of a radio frequencysputtering technique.

A still further advantage of the invention is the ability to coat anelectrode more efficiently and cost effectively through the use ofsolution casting techniques.

A still further advantage of the invention is the ability to moreadequately manipulate the microstructure of tungsten oxide thin filmsused as sensing films on electrodes for the sensing of hydrogen sulfidegas.

A very precise method for fabricating thin films of tungsten oxide hasbeen afforded by coupling the novel precursor compound with solutioncasting techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a chemiresistor sensor according to the presentinvention.

FIG. 2 is a schematic diagram showing a thin film of tungstencarboxylate precursor deposited on interdigitated electrodes.

FIG. 3 is a graphical representation of a typical relationship betweenhydrogen sulfide concentration and output voltage of a hydrogen sulfidegas sensor of the type shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the present invention willbecome more evident as the invention is more fully described herein.

The inventive compounds are novel tungsten carboxylates having theformulas I, II and III: ##STR2##

Because tungsten carboxylates of Formula I like many tungsten compounds,appear to exist as mixed-valence species (e.g. Formula I envisions oneW^(III) and one W^(IV) per unit; the formulas shown represent empiricalformulas and not structural formulas. Proposed structuralrepresentations of compounds in genus I and III are shown below,##STR3## but applicants do not wish to be restricted to such structures;compounds made by analogous methods and having the empirical formulas Iand III are intended to be encompassed within the invention.

Compounds of Formula II contain tungsten only in the +2 oxidation state.By analogy to the known tungsten (II) diacetate, they are believed morelikely to exist as dimers, or as straight-chain polymers of structureIIb than as monomers as shown in structure IIa. [See Holste, Z. Anorg.Allg. Chem. 398, 249-256 (1973).] ##STR4## Nonetheless structure IIaillustrates an important aspect of tungsten carboxylates: namely, thatthe tungsten-oxygen bonding cannot be represented as strictly a singlebond between one tungsten and two oxygens. Rather, as a result ofdelocalization of electrons through both oxygens of the carboxyl group,each tungsten is surrounded by four equivalent oxygens.

As shown in Formula IIb, there are probably metal-metal bonds betweenadjacent tungstens, and these bonds are thought to be double-bond innature. The structure IIb allows one to rationalize the lipid solubilityof the compounds of the invention: the "exterior" surface of the chainsbeing comprised entirely of hydrocarbon residues would allow the solventto interact extensively with the lipid R groups.

Tungsten carboxylates are exemplified wherein R is an aliphatichydrocarbon, such as C₇ H₁₅, but R can be any hydrocarbon chain, so longas the overall solubility and theological properties of the tungstencarboxylate in aliphatic or aromatic hydrocarbon solvents are notsignificantly changed. The preferred subgenera in which R is C₆ to C₁₀optimize the balance among solubility, rheology and reactivity of thestarting acid for forming the tungsten carboxylates. The novel mixedvalence tungsten (III) and (IV) carboxylates take the form of blueglassy solids and are sensitive to air and moisture. The tungsten (II)carboxylates of Formula II are dark green oils or glasses and are alsomoisture sensitive. The compounds of Formula II decompose in thepresence of moisture according to the reaction: ##STR5##

The compounds of Formulas I and III are believed to react similarly.

The solubility of the compounds in aliphatic and aromatic hydrocarbonsmakes them useful in solution casting techniques, such as spin-casting,dip-casting and spray-casting.

The inventive tungsten carboxylates may be synthesized according to thefollowing reactions: ##STR6##

The mechanism by which W^(VI) is reduced to W^(IV) and subsequently toW^(III) in equations 1 and 3 is speculated to involve oxidation of theligands to produce an olefin and carbon dioxide. In the case where R is1-ethylpentyl, the corresponding olefin, 3-heptene, has been identifiedamong the gaseous products of the reaction. In equations 1 and 3, R¹ andR² represent the appropriate alkyl, alkenyl or aralkyl residues thatwould arise from the corresponding R group according to the followingmechanism: ##STR7##

The method of preparing the compounds of Formula I comprises the stepsof reacting an alkali metal with an excess of an organic acid to form acarboxylate salt solution; reacting the carboxylate salt solution with asolution containing tungsten (VI) oxychloride in an aromatic solvent inan inert atmosphere to form a reaction mixture; refluxing the reactionmixture to form a sodium tungsten (III & IV) carboxylate, and extractingthe sodium tungsten (III & IV) carboxylate from the refluxed mixture.Suitable aromatic solvents include toluene and benzene. Suitable alkalimetals include sodium, potassium and lithium. Particularly suitableorganic acids include 2-ethylhexanoic acid and 4-phenylbutyric acid or3-phenylbutyric acid.

Compounds of generic Formula II are prepared in an inert atmosphere byheating tungsten hexacarbonyl with a large excess of the appropriatecarboxylic. acid at reflux, for acids with boiling points below 200° C.,or at 200° C., for those boiling higher. The heating is maintained untilall the tungsten hexacarbonyl is consumed. The solution is filtered andthe excess acid is distilled off under reduced pressure.

Compounds of generic Formula III are prepared by heating tungstenoxychloride with a large excess of the appropriate carboxylic acid in aninert atmosphere at about 160° C. The reaction is filtered and theexcess acid is distilled off under reduced pressure. Examples of samplepreparations of the inventive compounds are set forth below.

EXAMPLE 1 Formula I, R=1-ethylpentyl

Working in a conventional dry box, 3.43 g (10.0 mmol) of tungsten (VI)oxychloride was placed in a 200 ml Schlenk flask. Toluene (65 mL) wassyringed onto the sample. Freshly cut sodium (0.949 g, 41.3 mmol) wasplaced into a 250 mL 2-neck flask. The 2-neck flask was connected to theSchlenk flask by a bent elbow. The second neck of the 2-neck flask wasstoppered using a rubber septum. Outside of the dry box, 45 mL of2-ethylhexanoic acid was syringed onto the sodium and the mixture washeated below the boiling temperature of 113° C. until the sodium hadcompletely reacted. The adapter to the Schlenk flask was purged withnitrogen gas before opening the system to a connected bubbler. The2-ethylhexanoic acid-salt solution was added to the tungsten (IV)oxychloride solution while stirring at room temperature. Under a purgeof nitrogen gas, a condenser was connected to the Schlenk flask. Thereaction mixture was refluxed using an oil bath heated at 125° C. After16 hours, the solution was cooled to room temperature under a purge ofnitrogen. The toluene was removed by vacuum distillation. To remove theexcess 2-ethylhexanoic acid, a dynamic vacuum was used while heating at110° C. with an oil bath. The glassy blue product was extracted from thesodium chloride in the refluxed mixture with pentane.

EXAMPLE 2 Formula II R=1-ethylpentyl

Working outside of the dry box, 4.04 g (11.4 mmol) of W(CO)₆ was weighedand placed into a 240 mL Schlenk flask fabricated for refluxing reactionmixtures. Using a syringe, 80 mL (0.5 mol) of 2-ethylhexanoic acid (EHA)was added to the flask. After connecting a condenser that was attachedto a nitrogen line, the reaction flask was heated using an oil bath. Thetemperature of the oil bath reached 195°-200° C. Maintaining thetemperature the solution was heated until W(CO)₆ no longer sublimed upon the walls of the flask (4 days). A dark green solution was observed.Periodically the W(CO)₆ was washed down from the walls by agitating thesolution. Before the heat was removed, the valve on the Schlenk flaskwas closed to prevent air from going into the flask. Working in a drybox, the solution was filtered through a 0.45 micron cellulose acetatefilter. The filtered solution was transferred to a 200 mL round bottomsingle-neck flask which was connected to a 250 mL Schlenk flask using abent elbow. The excess EHA was removed by heating the dark greensolution with a 130° C. oil bath under vacuum. A dark green materialwith a thick oil consistency was obtained. The infrared spectrum of thematerial was consistent with product of Formula II where R is1-ethylpentyl (carbonyl at 1680 cm⁻¹).

EXAMPLE 3 Formula III, R=1-ethylpentyl

Working in the dry box 4.13 g (12.1 mmol) of OW Cl₄ was weighed into aSchlenk flask that was fabricated for refluxing reaction mixtures. Then,60 mL (0.375 mol) of 2-ethylhexanoic acid was added to the reactionflask. Working outside of the dry box, the nitrogen purged condenser wasattached to the reaction flask (the valve was still closed at thispoint). After heating the reaction flask in a 100° C. bath for fiveminutes, the valve was opened. The condenser was connected to a bubblerand the oil bath was heated to 160° C. The reaction mixture turned deepblue/purple. It was maintained at 160° C. for 24 hours. Before the heatwas removed, the valve on the reaction flask was closed to preventexposure to air. The reaction flask was taken into the dry box and thesolution was filtered through a 0.45μ cellulose nitrate filter. Usingpentane, the remaining material was rinsed out of the flask. A dark bluesolid was collected on the filter medium. The filtered solution wastransferred to a 250 mL one neck flask. The pentane was removed in vacuoand the excess acid was distilled off at 120° C. under vacuum.

The material was placed in a 250 mL one neck flask. The flask wasconnected to a fine porosity frit and a 250 mL Schlenk flask. About 100mL of diethyl ether was distilled onto the product in vacuo. Theether-soluble portion of the material was extracted into the 250 mLSchlenk flask. The dark blue solid product (Formula III R=ethylpentyl)was isolated by removal of the ether in vacuo.

The tungsten carboxylates are ideal precursors for providing a tungstenoxide thin film over electrodes used in hydrogen sulfide gas sensors. Itis well known that tungsten oxide is an ideal film for coating ofelectrodes in hydrogen sulfide gas sensors, because tungsten oxide filmshave shown good selectivity and sensitivity to hydrogen sulfide gas. Theresistance to a current passed through the chemiresistor comprised of atungsten oxide film coating on electrodes decreases when hydrogensulfide is in the ambient gas. The decrease in resistance is believed tobe caused by an exchange/reduction between O⁻² and S⁻² with theproduction of WS₂, which has a greater conductivity than WO₃. Theresulting exchange between O⁻² and S⁻² can be measured by an increase involtage at a detector device. This is accomplished by having the sensorconnected to a standard operational amplifier circuit incorporating thedetector device. The decrease in resistance translates into an increasein voltage which is relative to the concentration of the hydrogensulfide gas. FIG. 3 shows the relationship between the concentration ofthe hydrogen sulfide gas and the increase in voltage of the sensingdevice caused by the decreased resistance of the chemiresistor.

The present invention provides an improved film coating of tungstenoxide on the electrodes used in hydrogen sulfide gas sensors. The filmof the novel tungsten carboxylate precursor of the present invention isapplied or deposited on the electrodes, preferably arranged in aninterdigitated configuration, by a known solution casting technique.While it is not possible to solution cast tungsten oxide (because it isinsoluble in solvents typically used in solution casting processes), theinventive tungsten carboxylate precursors can be applied to electrodes,including interdigitated electrodes supported on an inert substrate bysolution casting techniques. This is possible because the novelcarboxylates are soluble in the solvents used in solution castingtechniques and have the necessary rheology and surface wettingproperties. The resulting thin films from II and III decompose totungsten oxide when heated to above approximately 350° C. byconventional curing methods; the films from I decompose to sodiumtungsten oxide.

The mechanism of the decomposition is not clearly established butappears to involve oxidation of tungsten by atmospheric oxygen, perhapsmediated through the ligand.

A general procedure for coating a substrate is provided by the followingexample:

Working in the dry box, a toluene solution containing the precursorcompound was syringed onto a quartz glass plate. The thin film was laiddown using a photoresist spinner at 2000 rpm for 20 seconds. The quartzglass was either placed on a hot plate or in an oven. After 30 minutes,the glass was removed and a transparent thin film was observed. Theresults from x-ray diffraction are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Compound Film Deposit                                                                             Heating                                                   of example #                                                                           Conditions Conditions (°C.)                                                                   Nature of film                                ______________________________________                                        1        17 mg/50 μl                                                                           500°/hot plate                                                                     sodium tungstate                              2        17 mg/50 μl                                                                           520°/hot plate                                                                     hexagonal phase                                                               WO.sub.3                                      2        40 mg/100 μl                                                                          300°/oven                                                                          amorphous WO.sub.3                            2        40 mg/100 μl                                                                          500°/oven                                                                          cubic phase WO.sub.3                                                          containing small                                                              amount of triclinic                           2        40 mg/100 μl                                                                          300°/hot plate                                                                     cubic phase WO.sub.3                                                          containing small                                                              amount of triclinic                           2        40 mg/100 μl                                                                          500°/hot plate                                                                     cubic phase WO.sub.3                                                          containing small                                                              amount of triclinic                           3         9 mg/100 μl                                                                          300°/oven                                                                          amorphous WO.sub.3                            3         9 mg/100 μl                                                                          500°/oven                                                                          partially crystal-                                                            line cubic phase                                                              WO.sub.3                                      3         9 mg/100 μl                                                                          300°/hot plate                                                                     amorphous WO.sub.3                            3         9 mg/100 μl                                                                          500°/hot plate                                                                     partially crystal-                                                            line triclinic phase                                                          WO.sub.3                                      ______________________________________                                    

The results demonstrate that not only can a coated tungsten oxideelectrode be produced by thin film casting techniques, but themicrostructure of the film can be modulated by changing the precursorand the heating conditions.

FIG. 1 shows a sensor 22 of the present invention having a substratesupport layer 10 made from inert materials, such as quartz andcontaining or having mounted thereon conductors of a conductor layer 11,12, 13, 14. The conductors 11, 12, 13 and 14 are made from conductingmaterial, such as gold or palladium. Electrical current can be passedfrom a standard power supply via a conducting wire or other meansthrough the conducting layer 11. The conducting layer 11 is in contactwith an adjacent resistor or heater layer 15. The resistor layer 15generates heat from the conducted current. The current is then passedfrom the resistor layer 15 to the conductor 14 and via a standardconducting means back to a power supply.

On the upper side of the resistor layer 15, there is a silicon-oxidebased dielectric layer 16, upon which there is mounted an electrodelayer comprising electrodes 17 and 17'. A sensing film 18 according tothe instant invention is deposited over the electrodes 17 and 17'.

The dielectric layer 16 functions in the sensor 22 to shield theresistor layer 15 from reacting directly with the sensing film 18.

The resistor layer 15, heats the coated electrodes 17 and 17' to improvesensitivity and selectivity of the sensing film 18, as is commonly donein gas sensor technology.

Electrical current is also passed from a power supply through aconductor 12, to the electrode 17. The current transfers to electrode17' and is passed through the conductor layer 13 which is connected to astandard operational amplifier circuit with a detector means, of thetype known in the art.

The electrode layer 17 and 17', is preferably arranged as interdigitatedelectrodes which have been coated with the sensing film 18, using asolution casting technique. The sensing film 18, is a tungsten oxide orsodium tungsten oxide thin film formed from thermally decomposing thenovel tungsten carboxylate compounds. The sensing film 18, selectivelyreacts with hydrogen sulfide gas in the ambient atmosphere to cause anincrease in the conductance of a current passed through the electrodes17 and 17'.

FIG. 2 shows a schematic representation of the thin sensing film 18,deposited on an interdigitated array of conducting electrodes 17 and17'.

FIG. 3 is a graphical representation of a typical relationship betweenhydrogen sulfide concentration and output voltage of a hydrogen sulfidegas sensor 22 of the type shown in FIG. 1 when coated according to thegeneral procedure with the compound of example 1 and heated at 500° C.The trend shows that there is an increase in output voltage with anincrease in hydrogen sulfide gas concentration.

It is believed that tungsten oxide reacts with hydrogen sulfide gas toform tungsten sulfide.

It is also believed that the introduction of oxygen gas in the absenceof H₂ S promotes the resulting tungsten sulfide (WS₂) to reform thetungsten oxide film.

    WS.sub.2 +7/2 O.sub.2 →WO.sub.3 +2SO.sub.2

Sensors fabricated according to the present invention have improvedsubstrate conformity, a more uniform doping ability, less potentialstress in the films and are more conveniently fabricated than those madeby known methods.

Accordingly, the preferred embodiments of the invention have beenillustrated and described in detail. It is to be understood thatnumerous changes and variations can be made in the composition andmanufacture of the invention without departing from the spirit of theinvention, the scope of which is defined by the appended claims.

We claim:
 1. A tungsten salt, soluble in aprotic solvents, prepared by the process consisting essentially of combining tungsten (VI) oxychloride with four equivalents of sodium 2-ethylhexanoate in the presence of a large excess of 2-ethylhexanoic acid in toluene and refluxing for 16 hours.
 2. A compound of formula ##STR8## wherein n is from zero to three.
 3. Tungsten bis(2-ethylhexanoate) according to claim
 2. 4. A tungsten compound, soluble in aprotic solvents, prepared by the process consisting essentially of combining tungsten (VI) oxychloride with thirty equivalents of 2-ethylhexanoic acid and heating at 160° C. for 24 hours.
 5. A method for preparing sodium tungsten oxide which comprises thermally decomposing a compound of formula

    Na[OW(OOCR).sub.2 ].sub.2

wherein R is alkyl, alkenyl or aralkyl of 2 to 19 carbons.
 6. A method of preparing sodium tungsten oxide according to claim 5 wherein R is a 1-ethylpentyl radical.
 7. A method of preparing a compound of formula

    Na[OW(OOCR).sub.2 ].sub.2

wherein R is alkyl, alkenyl or aralkyl of 2 to 19 carbons, which comprises: reacting an alkali metal with an excess of a C₃ to C₂₀ acid to form an acid-salt solution in said acid; reacting said acid-salt solution with a solution containing tungsten (VI) oxychloride in an aromatic solvent to form a sodium tungsten carboxylate. 